To save energy, for example, PTL 1 proposes a hermetic compressor. In the hermetic compressor, a cylinder is provided with a columnar cylinder chamber. An oil supply hole is formed in an upper wall of the cylinder, and communicates the inside and outside of the cylinder chamber with each other. An annular groove is formed in an outer peripheral face of a columnar piston.
In such a hermetic compressor, the piston reciprocates in the cylinder chamber. When the piston is located at the bottom dead center, the oil supply hole communicates with an annular groove. When the piston is located at the top dead center, the oil supply hole communicates with the cylinder chamber. Thus, lubricating oil passes through the oil supply hole, and fills a gap between the outer peripheral face of the piston and the inner peripheral face of the cylinder chamber. This lubricating oil lubricates the portion between the piston and the cylinder chamber, and seals the cylinder chamber.